Conventionally, spark plugs which ignite air-fuel mixtures by spark discharge have been used for ignition plugs of engines which are internal combustion engines of automobiles. In recent years, higher power outputs and lower fuel consumptions have been demanded of such internal combustion engines. Because of this, progress has been made in the development of plasma-jet spark plugs that can ignite leaner air-fuel mixtures which burn out quickly and whose ignitable limit air-fuel ratios are higher.
For example, Japanese unexamined patent application publication No. JP-A-2007-287666 describes a related art plasma-jet spark plug. The related art plasma-jet spark plug has a structure in which a cavity, having a small capacity, is formed as a discharge space by surrounding the periphery of a spark discharge gap, between a center electrode and a ground electrode, with an insulator.
The related art plasma-jet spark plug has been manufactured by taking, in general, the following steps (1) to (3). (1) A plate-shaped ground electrode, in which a through hole is formed in a center, is press fit in a ground electrode mounting portion provided at a leading end of a metal shell with a predetermined fitting tolerance. (2) The metal shell and the ground electrode are laser welded together. (3) An insulator, in which a center electrode is built in advance, is held within the metal shell to which the ground electrode has been welded by crimping the insulator to a predetermined engagement portion.
However, in the manufacturing method described above, there was a possibility that the insulator was pressed against the ground electrode with a pressure larger than required when the insulator was made to be held within the metal shell. Therefore, a slight gap was provided between the insulator and the ground electrode, so as to solve the problem. However, in the event that the gap is provided between the insulator and the ground electrode, the energy held by plasma leaks into the gap, leading to a concern that ignitability is reduced.